Report an error for an implicit implementation of an inaccessible interface members

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -7124,4 +7124,7 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="ERR_MisplacedUnchecked" xml:space="preserve">
     <value>Unexpected keyword 'unchecked'</value>
   </data>
+  <data name="ERR_ImplicitImplementationOfInaccessibleInterfaceMember" xml:space="preserve">
+    <value>'{0}' does not implement interface member '{1}'. '{2}' cannot implicitly implement an inaccessible member.</value>
+  </data>
 </root>

src/Compilers/CSharp/Portable/Errors/ErrorCode.cs

@@ -2088,6 +2088,7 @@ internal enum ErrorCode
         ERR_UnsupportedCompilerFeature = 9041,
         WRN_ObsoleteMembersShouldNotBeRequired = 9042,
         ERR_RefReturningPropertiesCannotBeRequired = 9043,
+        ERR_ImplicitImplementationOfInaccessibleInterfaceMember = 9044,
 
         #endregion
 

src/Compilers/CSharp/Portable/Symbols/Source/SourceMemberContainerSymbol_ImplementationChecks.cs

@@ -1609,7 +1609,7 @@ private void CheckInterfaceUnification(BindingDiagnosticBag diagnostics)
         {
             foreach (Diagnostic diagnostic in implementingMemberAndDiagnostics.Diagnostics.Diagnostics)
             {
-                if (diagnostic.Severity == DiagnosticSeverity.Error && diagnostic.Code is not (int)ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember)
+                if (diagnostic.Severity == DiagnosticSeverity.Error && diagnostic.Code is not ((int)ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember or (int)ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember))
                 {
                     return default;
                 }

src/Compilers/CSharp/Portable/Symbols/TypeSymbol.cs

@@ -996,23 +996,44 @@ private static Symbol ComputeImplementationForInterfaceMember(Symbol interfaceMe
             {
                 if ((object)implicitImpl != null)
                 {
-                    if (!canBeImplementedImplicitlyInCSharp9)
+                    bool suppressRegularValidation = false;
+
+                    if (!canBeImplementedImplicitlyInCSharp9 && interfaceMember.Kind == SymbolKind.Method &&
+                        (object)implementingBaseOpt == null)  // Otherwise any appropriate errors are going to be reported for the base.
                     {
-                        if (interfaceMember.Kind == SymbolKind.Method &&
-                            (object)implementingBaseOpt == null) // Otherwise any approprite errors are going to be reported for the base.
+                        var useSiteInfo2 = compilation is object ? new CompoundUseSiteInfo<AssemblySymbol>(diagnostics, compilation.Assembly) : CompoundUseSiteInfo<AssemblySymbol>.DiscardedDependencies;
+
+                        if (implementingType is NamedTypeSymbol named &&
+                            !AccessCheck.IsSymbolAccessible(interfaceMember, named, ref useSiteInfo2, throughTypeOpt: null))
+                        {
+                            diagnostics.Add(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, GetImplicitImplementationDiagnosticLocation(interfaceMember, implementingType, implicitImpl), implementingType, interfaceMember, implicitImpl);
+                            suppressRegularValidation = true;
+                        }
+                        else if (!interfaceMember.IsStatic)
                         {
                             LanguageVersion requiredVersion = MessageID.IDS_FeatureImplicitImplementationOfNonPublicMembers.RequiredVersion();
                             LanguageVersion? availableVersion = implementingType.DeclaringCompilation?.LanguageVersion;
                             if (requiredVersion > availableVersion)
                             {
-                                diagnostics.Add(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, GetInterfaceLocation(interfaceMember, implementingType),
+                                diagnostics.Add(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, GetImplicitImplementationDiagnosticLocation(interfaceMember, implementingType, implicitImpl),
                                                 implementingType, interfaceMember, implicitImpl,
                                                 availableVersion.GetValueOrDefault().ToDisplayString(), new CSharpRequiredLanguageVersion(requiredVersion));
                             }
                         }
+
+                        diagnostics.Add(
+#if !DEBUG
+                            // Don't optimize in DEBUG for better coverage for the GetInterfaceLocation function. 
+                            useSiteInfo2.Diagnostics is null ? Location.None :
+#endif
+                            GetImplicitImplementationDiagnosticLocation(interfaceMember, implementingType, implicitImpl),
+                            useSiteInfo2);
                     }
 
-                    ReportImplicitImplementationMatchDiagnostics(interfaceMember, implementingType, implicitImpl, diagnostics);
+                    if (!suppressRegularValidation)
+                    {
+                        ReportImplicitImplementationMatchDiagnostics(interfaceMember, implementingType, implicitImpl, diagnostics);
+                    }
                 }
                 else if ((object)closestMismatch != null)
                 {